1. Field of the Invention
The invention relates to implantable prostheses that are suitable for replacement of diarthroidal or arthroidal joints by creating an artificial diarthroidal-like joint at the site of the implant.
In a particular embodiment, the invention relates to implantable prostheses serving as replacements for at least a portion of the intervertebral disc material, i.e., a spinal disc endoprostheses suitable for implantation in vertebrates, including humans.
2. Description of Related Art
Many joints in the human body, such as hips, knees, shoulders, etc., are diarthroidal, meaning that the joints include a joint capsule that is filled with fluid. The capsule fluid lubricates the joint, and allows the surfaces of the joint to move with a low coefficient of friction. The spine, by contrast, can be considered to be a series of joints, some of which (the anterior joint or disc) lack a fluid filled capsule and are therefore arthroidal (the spine also contains facet joints that are diarthroidal). The interior portion of intervertebral discs are not provided by the body with significant blood supply; their homeostasis is enhanced by the diffusion of fluids into the disc tissue, thus supplying them with nutrients. This, to some extent, allows the tissue to grow and repair damage done by stress as the joint moves. Despite this process in mature adults, spinal disc tissue degrades continuously over time. Sufficiently advanced degeneration can lead to herniation or rupture of the spinal disc.
Herniation of a spinal disc can result in a number of debilitating symptoms, including intractable pain, weakness, and sensory loss. Treatment of these symptoms frequently requires surgical removal of at least a portion of the herniated disc, a procedure known as discectomy. Often discectomy alone cannot stop the progressive degeneration at the level of disc excision. An additional procedure is often performed in conjunction with the discectomy with the objective of fusing together (arthrodesis) the vertebral bodies surrounding the affected disc space. This is accomplished by removing the cartilaginous endplates by scraping the surfaces of the vertebral body and inserting a piece of graft bone, which may be an allograft from a bone bank, or an autograft, typically taken from the iliac crest of the patient, or other suitable material.
The discectomy and arthrodesis procedures can be problematic, however. Discectomy problems have been described above. The grafting or fusion procedure has a variable success rate of about 80%, and even when successful, requires considerable recovery time before fusion is complete. Perhaps of even greater concern, successful fusion eliminates normal spinal biomechanics. Range of motion at the level of the fusion is ideally eliminated, because the affected vertebrae have been effectively joined to form a single bone. Because the patient tries to maintain the same overall range of motion of the entire spine, additional stress is imposed on the intervertebral discs of the adjacent vertebrae. This, in turn, may lead to accelerated degeneration at levels above and below the fusion site, which may require additional treatment, including discectomy and fusion. Grafting procedures carry some risk of tissue rejection and disease transmission if an allograft is used, and risk of harvest site morbidity when the patient's own tissue is harvested.
As a result of these difficulties with intervertebral fusion, attempts have been made to provide a prosthetic solution to degenerative disc disease that maintains the patient's normal spinal biomechanics, allows for shorter recovery times, and avoids the complications inherent in harvesting and/or grafting bone tissue. Some of these efforts have centered around providing an endoprosthetic intervertebral implant, as described in U.S. Pat. Nos. 5,865,846, 5,674,296, 5,989,291, 6,001,130, and 6,022,376, the entire contents of each of which is hereby incorporated by reference.
Design and construction of such an implant, however, is not simple. Desirably, the implant should be precisely placed in a prepared intervertebral space, and should contain elements that are immobilized with respect to each of the vertebral bodies, so that the implant does not migrate or shift, potentially contacting, abrading, or otherwise damaging the spinal cord, ligaments, blood vessels, and other soft tissue. At the same time, the implant should allow the vertebral bodies to move relative to each other in a way that provides the equivalent motion afforded by a healthy intervertebral disc, and that allows the affected vertebral joint to participate in the coordinated overall movement of the spine in a way that closely approximates the natural movement of a healthy spinal column. The implant should be biocompatible, and avoid the introduction of toxic or harmful components into the patient, such as release of wear debris. The implant should also restore normal disc height and maintain the patient's vertebral lordosis, and should not allow any significant post-operative subsidence. The implant should be at least partially constrained by soft tissue in and around the intervertebral space, in order to allow a simpler, more efficient design. There remains a need for a device which would decrease patient recovery time, and reduce the occurrence of postoperative degeneration at levels above and below the implant, as compared with fusion techniques. In addition, such an implant would avoid the need for harvesting of autograft bone tissue, thereby eliminating morbidity at the harvesting site. Such an implant should also provide elasticity and damping sufficient to absorb shocks and stresses imposed on it in a manner similar to that of the natural spinal disc.